Is Asteroid 2002Ny40 a Rubble Pile Gravitationally Disrupted? J

Lunar and Planetary Science XXXIX (2008) 1692.pdf

IS ASTEROID 2002NY40 A RUBBLE PILE GRAVITATIONALLY DISRUPTED? J. M. Trigo-Rodríguez1,2, W.F. Bottke3, A. Campo Bagatin4, P. Tanga5, J. Llorca6, D.C. Jones7, I.P. Williams7, J.M. Madiedo8, and E. Lyytinen9. 1Institute of Space Sciences (CSIC). Campus UAB, Facultat de Ciències, Torre C5-2ª planta. 08193 Bellaterra, Spain; 2Institut d’Estudis Espacials de Catalunya. Gran Capità 2-4, Ed. Nexus. 08034 Bar- celona, Spain; 3 Southwest Research Institute, Boulder, Colorado, USA; 4 Universidad de Alicante, Spain; 5 Obser- vatoire de Nice, France; 6 Institut de Tècniques Energètiques, Universitat Politècnica de Catalunya (UPC), Barce- lona; 7 Astronomy Unit, Queen Mary University of London, London, United Kingdom; 8 Facultad de Ciencias, Uni- versidad de Huelva, Spain; 9 Finish Fireball Network, Kehäkukantie 3B, 00720 Helsinki, Finland.

Introduction: The existence of meteoroid streams Results and discussion: Due to the relative low containing meter-sized meteoroids capable of produc- population of the NEO region, a collision among as- ing meteorites after atmospheric interaction was pro- teroids is an unlikely, but not negligible process for posed quite recently [1]. Their existence has important producing the observed asteroidal fragments. In any implications because they can be naturally delivering case, we have searched for alternative processes capa- to the Earth different types of rock-forming materials ble of producing the meteoroid stream. a) One option from Potentially Hazardous Asteroids (PHA). would be that the meteoroids were ejected by a fast rotator, unlikely according to the present spin of both The recent identification of Near Earth Object asteroids, but not taking into account the YORP effect. (NEO) asteroid 2002NY40 as source of meteorite- We should remark that the original spin of the progeni- dropping bolides [2] opens new questions on the na- tor asteroid would have been very different. b) An- ture of this asteroid, and the physical process that other possibility for the origin of this complex would originated the fireballs detected in 2006 August. Two be a catastrophic disruption where typically the escape of the fireballs exhibited a clear similarity with the velocity is considerably smaller than the orbital veloc- orbit of 2002NY40, while a third meteoroid was hav- ity, producing a large amount of the mass being ing an orbit close to NEO 2004NL8. In fact, these five ejected away [4]. In this sense, a gravitational-induced bodies would be related by the close similarity among disruption during a close approach of a progenitor asteroid to the Earth. In fact, both NEOs are approaching their orbits (Figure 1, and Table 3 of [1]). We try to significantly to the Earth, and Mars. In support of this focus here on the possible origin of this complex of idea, the structure of 2002NY40 has been recently bodies on the basis of our present (little) knowledge on reconstructed during its last approach to the Earth, just the structure and orbital evolution of asteroids revealing an irregular triangular shape [5]. Would be 2002NY40 and 2004NL8. this asteroid a reaccumulated rubble pile?. If so, the weakness of such structure under a close approach to Methods: The identification of fireball activity the Earth or Mars may lead to disruption. Such mecha- from 2002NY40 was achieved during 2006 by the nism would link the two NEOs to a larger progenitor, Finish Fireball Network (FN) and the Spanish Meteor and the fragments would be small pieces released dur- Network (SPMN). The locations of FN and SPMN ing the encounter. The orbital elements of both NEOs stations, trajectory and orbital data of the three fire- would support such a picture. 2002NY40 and balls were already given in [1]. Astrometric reduction 2004NL8 asteroids have very close values of argument was performed by using the common software that we of perihelion (5.884º and 4.93º, respectively), and ec- are using for trajectory and orbital determination of centricity (0.7098 and 0.7203). Moreover their orbits meteoroids. We also studied the fireball spectra of two (and those of the identified meteoroids) are almost of the bolides following the procedures described in coplanar. We plan to check these possibilities by mean [3]. From such approach we estimated the expected of a N-body code-based simulations. c) A collisional mineralogy of the fireballs, that was compared to the fragmentation of a progenitor with a main belt asteroid reflectance data obtained for NEO 2002NY40. is also possible, due to the high eccentricity of both objects that drive them inside that region periodically. The theoretical probability of collision with an asteroid during one of the objects’ enters into the main From the point of view of the mineralogy of belt has been calculated. The fraction of D > 1 km NEOs 2002NY40, spectrophotometric data of this asteroid that disrupt by hitting a main belt object is a few percent. A has recently revealed its possible mineralogical link relatively small, but not negligible value that we prefer to with LL ordinary chondrites [5,6]. In this sense, we keep for a more detailed study. measured from fireball video spectra (Fig. 1) the chemical abundances of the main elements of the two bolides [2]. Quite interestingly, we found relatively Lunar and Planetary Science XXXIX (2008) 1692.pdf

high Mg/Fe ratios (Mg/Fe=1.2±0.1), and Na/Fe=0.05±0.01 that are close to the LL chondritic mineralogy [7]. This suggests that the 2002NY40 complex (including perhaps 2004NL8) would be one of the present sources of LL chondritic meteorites that we find in terrestrial collections.

Interestingly, many objects coming out of the inner main belt (already in the NEO population) appear to have this taxonomic type. These objects are probably

part of a "train" of objects coming out of the ν6 secular resonance or other small mean motion resonances in the inner main belt. Consequently, such mechanism Figure 1. The order zero image and spectrum of the Sept. 10, 2006 Finish fireball (FN100906). Its heliocentric orbit would be an efficient source of LL-chondrites to the suggests the origin of this rock in NEO 2004NL8 [2]. Earth.

In conclusion, we think that the present case de- serves a future detailed study that is in progress. In any case, this is a nice example that the orbital evolution of meteoroids, and their chemical composition can pro- vide interesting clues on the evolution of NEO objects in the Earth’s vicinity (Fig. 2). In this case, three bright bolides have pointed this out. We plan to perform ad- ditional modelling in order to better explain the origin of this interesting complex of minor bodies.

References: [1] Halliday I., Griffin A.A., and Blackwell A.T. (1996) Meteorit. & Planet. Sci. 31, 185-217. [2] Trigo- Rodríguez J.M., et al. (2007) Mon. Not. R. Astron. Soc. 382, 1933-1939. [3] Trigo-Rodríguez J.M., J. Llorca, J. Borovička

and J. Fabregat (2003) Meteorit. & Planet. Sci. 38, 1283- 1294. [4] Bottke W. et al. (2005) Icarus 175, 111-140. [5] Figure 2. The orbits of NEOs 2002NY40 and 2004NL8, Roberts L.C. et al. (2007) Icarus 192, 469-474. [6] Rivkin showing their close similarity with the three bolides recorded A.S. et al. (2003) LPSC XXXIV, abst. #1722. [7] Jarosewich by the Finish Network and the Spanish Meteor and Fireball E. (1990) Meteoritics 25, 323-337. Network (SPMN). The orbits of the terrestrial planets are shown for comparison.